Control systems



Nov. 7, 1967 F. J. HAYES 3,351,294

CONTROL SYSTEMS Filed Dec. 17, 1964 2 Sheets-Sheet l 22 22 2 22 V -222 E g :1, I8 21 i I 240 121 "24A Iwvavwroe E. F. J. HAYES B! V ATw-oe'uzvs United States Patent 3,351,294 CONTROL SYSTEMS Eric Frederick John Hayes, Stockton-on-Tees, England, assignor to Milltronics Limited, Toronto, Ontario, Canada Filed Dec. 17, 1964, Ser. No. 418,962 Claims priority, application Great Britain, Dec. 17, 1963, 49,691/63 Claims. (Cl. 241-34) This invention relates to control systems, particularly for grinding wheels for grinding ore, cement, sandstone and like materials.

Grinding wheels, in which the material to be ground is continuously fed into the mill and the ground material is continuously removed have been previously suggested in a paper entitled, Making Rock Grind Itself, in the Engineering and Mining Journal, June 1955 and in patent specification No. 632,532. It has also been proposed as in British patent specification No. 782,523 to control the rate of feed to the mill by the noise generated by the mill and detected by a microphone located adjacent to the mill.

In the present invention, we employ a force responsive device, such as a loadcell, arranged in the support for the mill or its drive and adapted to control the supply of material to, or the withdrawal of material from, the mill. Thus the force responsive device may measure the weight of the mill, including its contents, or the torque supplied by the drive, and may operate to keep constant the indicated weight of the contents or the driving torque.

Thus in accordance with the present invention a grinding mill system comprises a rotary grinding mill, driving means for the mill, means for continuously supplying material to be ground to the mill, means for continuously withdrawing ground material from the mill, force responsive means arranged in the support for the mill and/ or the driving means and control means controlled by the force responsive means for controlling the supplying means or withdrawing means.

The invention will be more readily understood by way of example from the following description of an autogenous grinding mill of the type described in British patent specification No. 632,532, and the control system for the mill, reference being made to the accompanying drawings, in which FIGURES 1 and 2 are a side view and a plan view re spectively of the mill and drive, and

FIGURE 3 schematically illustrates the control system.

The mill, as described in the beforementioned specification No. 632,532, consists of a drum 12 having trunnions 13 supported in trunnion bearings 14 for rotation of the mill drum about a horizontal axis. The trunnion bearings 14 are carried through brackets 15 on a bed plate 16 which is cut away to enable the lower part of the drum 12 to pass therethrough. The bed place 16 also carries an electric motor 17 which drives a reduction gear box 18, having an output pinion 20 coupled to the mill drum 12 through a continuous chain 21.

The bed plate 16 is supported through columns 22 by a horizontal platform 23 which is in turn carried on four loadcells 24A-24D disposed under the four corners of the platform 23. These loadcells are well known force transducers, which consist essentially of resistive strain gauges, and which when appropriately energised, give electric signals dependent on the forces to which the loadcells are subjected.

Turning to FIGURE 3, the ore or other material to be ground is fed from a stock pile or hopper to a belt conveyor, diagrammatically shown at 25, driven at variable rate by virtue of a control circuit 26. The conveyor 25 delivers the material to the drum 12 through one of the trunnions 13, while the ground material is withdrawn through the other trunnion 13 in a conduit 27 through which air is sucked by a fan (not shown); this air conveyance simultaneously ensures that only material ground to a given particle size is withdrawn and causes a degree of drying of the material.

The outputs of the four loadcells 24A-24D are coupled together to give a resultant signal representing the total Weight of the platform 23 and all the equipment carried by it and this resulting signal is applied to an electronic amplifier 28 which is backed off by the weight of everything supported by the loadcells 24, except the material carried by the drum 12; as a result the amplified signal from amplifier 28 represents the weight of the ore or other material within the drum 12. The signal from the amplifier is applied to a circuit 30 to which is also applied an electric signal on line 31, representing the desired weight of material in the drum. The difierence or error signal from the circuit 31, representing the departure of the material weight from the desired weight is amplified in amplifier 32 and applied to the control circuit 26 to control the rate at which material is supplied to the drum 12 by the conveyor 25. The system thus operates to keep the mass of the material in the drum 12 at any time approximately constant at the weight set on line 31 by a potentiometer or other manually controllable device.

While it is preferred to control the rate of supply of material as described, if desired the rate of removal of ground material from the drum 12 may alternatively be controlled by the signal 32. Also, as in this embodiment it is required only to obtain a measure of the weight of the material in the drum 12, the loadcells 24 may be arranged beneath the drum 12 only, i.e. the drive motor 17 and gear box 18 may be independently supported. Again, the drum may be supported by rollers mounted in a supporting structure and engaging with peripheral tracks on the drum and, in this case, the loadcells may be located between the roller mountings and supporting structure again to provide a measure of the weight of the material within the drum.

The mass of material Within the drum may also be kept approximately constant by controlling the driving torque applied to the drum 12. For this purpose, the outputs of loadcells 24A, 24B are summed, and the outputs of loadcells 24C, 24D are similarly summed. The sum signals are then ditferenced and applied as an input to the amplifier 28 as before. In this case, the datum signal applied on line 31 represents the desired value of torque, and the control system operates to keep the torque, and the control system operates to keep the torque, represented by the differenced signal, substantially constant at the value set on line 31. By the use of switches between the loadcells 24 and the amplifier 28, the mill may be optionally controlled with material weight or torque as the control parameter.

Where torque only is to be used as the control parameter, the mill 12 and drive motor 17 are preferably mounted independently of one another and the loadcells 24 are arranged to support only the motor; a more -accu rate torque-indicating signal, which is applied to control the circuit 26 as before, is then obtainable compared with that derived from the arrangement of the drawings.

The size of the feed entering the mill may also be indicated by continuously comparing the peak values of theoutput signal from amplifier 28 with the reference signal on line 31.

The mill may also be controlled by measuring the short term variation in the signal from amplifier 28. This signal has a DO component, due to the mass of material within the mill, with a short term random variation superimposed on it, due to the fall of large pieces of ore or other material within the mill. As the material content of the mill increases, the mill becomes more choked and the effect of the fall of the material lessens, so that the short term variation or AC. component of the amplifier signal is an indication of the loading of the mill. In this form of control then, the signal from amplifier 28 is first difierentiated to remove the DC. component, then rectified and finally integrated to give a control signal which is compared with a reference signal and employed to control the control circuit 26 as before.

I claim:

1. A grinding mill system comprising a rotary grinding mill, driving means for the mill, means for continuousy conveying material to be ground to the mill, means for continuously withdrawing ground material from the mill, force responsive means arranged to give a signal representing the force applied by the mill and the driving means to its support and control means operated by the signal from the force responsive means, said force responsive means being so arranged that the signal represents the torque applied by the driving means to the mill, the control means operating to maintain that signal substantially constant, for controlling the conveying means to maintain substantially constant the mass of material within the mill.

2. A grinding mill system according to claim 1 in which the force responsive means comprise a plurality of loadcells supporting the driving means.

3. A grinding mill system according to claim 1 in which the force responsive means comprise a plurality of loadcells supporting both the mill and the driving means.

4. A grinding mill system according to claim 1 in which there is also a circuit for indicating the short term variations in the signal from the force-responsive means.

5. A grinding mill system comprising a rotary grinding mill, driving means for the mill, means tor continuously supplying material to be ground to the mill, means for continuously withdrawing ground material from the mill, force responsive means arranged in the support for the mill and the driving means, said force responsive means giving an output signal in accordance with the force applied by the mass of the mill and its contents and the control means arranged to derive from the output signal a control signal dependent on short-term variations in the output signal and to control the supplying means in accordance with the departure of the mean value of the variations from a given value.

References Cited UNITED STATES PATENTS 2,764,360 9/1956 Podszus 241-34 3,253,744 5/1966 MacPherson et al 24134 OTHER REFERENCES Publication: Canadian Mining Journal; March 1961, pp. 6772.

ANDREW R. JU HASZ, Primary Examiner. G. A. DOST, Assistant Examiner. 

1. A GRINDING MILL SYSTEM COMPRISING A ROTARY GRINDING MILL, DRIVING MEANS FOR THE MILL, MEANS FOR CONTINUOUSY CONVEYING MATERIAL TO BE GROUND TO THE MILL, MEANS FOR CONTINUOUSLY WITHDRAWING GROUND MATERIAL FROM THE MILL, FORCE RESPONSIVE MEANS ARRANGED TO GIVE A SIGNAL REPRESENTING THE FORCE APPLIED BY THE MILL AND THE DRIVING MEANS TO ITS SUPPORT AND CONTROL MEANS OPERATED BY THE SIGNAL FROM THE FORCE RESPONSIVE MEANS, SAID FORCE RESPONSIVE MEANS BEING SO ARRANGED THAT THE SIGNAL REPRESENTS THE TORQUE APPLIED BY THE DRIVING MEANS TO THE MILL, THE CONTROL MEANS OPERATING TO MAINTAIN THAT SIGNAL SUBSTANTIALLY CONSTANT, FOR CONTROLLING THE CONVEYING MEANS TO MAINTAIN SUBSTANTIALLY CONSTANT THE MASS OF MATERIAL WITHIN THE MILL. 